The present invention generally relates to printing and particularly relates to methods and stamps for transferring patterns to a substrate in the presence of a third medium.
Printing thin layers of ink or other material from a patterned surface is well known in the printing industry. Printing processes were originally developed for the exchange and storage of information adapted to human vision. This typically requires pattern and overlay accuracies down to 20 μm for acceptable reproduction. Printing processes have been used for other forms of patterning. For example, gravure offset printing has been used to make 50-μm-wide conductor lines on ceramic substrates and to pattern thin-film transistors in low cost display devices. Offset printing has been used for fabrication of capacitors and metal conductor lines as narrow as 25 μm. Additionally, printed circuit board and integrated circuit packaging are popular applications of screen printing in the electronics industry. See, for example, B. Michel et al., IBM J. Res. Develop. 45, 697 (2001) and references therein.
Another conventional printing process is known as flexography. In flexography, a viscous ink is printed onto permeable materials such as porous paper, permeable plastic, and the like. Flexography is a rotary printing method involving resilient relief image plates to print images on materials that are difficult to print on with offset or gravure processes. Examples of such materials include cardboard, plastic films and substrates. Flexography is therefore used widely in packaging. Usually, the viscous ink prevents direct contact of the stamp with the substrate because it cannot be displaced quickly enough during fast printing operations. Transfer of a thick layer of ink is usually desired. However, this prevents replication of small feature sizes, typically smaller than 20 μm. See, for example, H. Kipphan, “Handbuch der Printmedien”, Springer Berlin, 2000 and J. M. Adams, D. D. Faux, and J. J. Rieber, “Printing Technology 4th Ed.”, Delamare Publishers, Albany, N.Y.
Micro contact printing uses a similar stamp to that used in flexography, but typically transfers a monolayer of ink onto an impermeable surface. A more general process called soft lithography has been applied to printing thiols and other chemicals onto a range of surfaces. Typically, the chemicals are first applied to the stamp as solutions in a volatile solvent or via a contact inker pad. After inking and drying, molecules in the bulk and surface of the stamp are in a “dry” state. The molecules are transferred by mechanical contact. The stamp is typically formed from poly-(dimethyl)siloxane (PDMS). See, for example, B. Michel et al. “Printing meets lithography”, IBM, J. Res. Develop. 45 (5), 697 (2001)).
Micro contact processing, soft lithography, and flexography involve locally defined, intimate contact without voids between stamp and substrate. This is generally known as conformal contact. Conformal contact comprises macroscopic adaptation to the shape of the substrate and microscopic adaptation of a soft polymer layer to a rough surface.
Micro array technology is expected to accelerate genetic analysis. Micro arrays are miniature arrays of gene fragments or proteins attached to or deposited on glass chips. These so-called “biochips” are useful in examining gene activity and identifying gene mutations. A hybridization reaction is typically used between sequences on the micro array and a fluorescent sample. In a similar manner, protein markers, viruses, and protein expression profiles can be detected via protein specific capture agents. After reaction, the chip is read with fluorescence detectors. The intensity of fluorescing spots on the chip is quantified. The demand for micro arrays and techniques for fabricating micro arrays is increasing. Conventional methods for patterning biological molecules onto biochips are described, for example, in M. Schena, “Micro array Biochip Technology”, Eaton Publishing, Natick Mass., (2000). In a first conventional method, a surface is treated with compounds in a sequential manner by: pipetting with a pipetting robot or capillary printing; dispensing droplets with an ink jet; or, patterning with a pin spotter. In a second conventional method, a surface is patterned with molecules in parallel thus reducing manufacturing cost. Microfluidic networks, capillary array printing, or micro contact processing can be employed in implementation of the second method.
The printing of biological molecules and water soluble catalysts by conventional techniques does not always work, is difficult to reproduce, and results are variable. Repetitively creating homogeneous prints with high yield over large areas is very difficult, particularly if the molecules require permanent hydration. See, for example, A. Bernard et al., “Micro contact Printing of Proteins”, Adv. Mater. 2000 (12), 1067 (2000). Many biological molecules require at least partial hydration. Also, many biological processes operate only when there is liquid to provide mobility. When molecules are to selectively perform chemical reactions on a surface in a patterned fashion, it is desirable to fix the molecules in place to avoid blurring the pattern by spreading. In catalytic printing therefore, it would be desirable to tether molecules so that they can reach the surface only where desired. Limited mobility should be permitted so that molecules can function effectively without escaping. Biological molecules preferably encounter the substrate while immersed in a layer of water to permit a chemisorption reaction. Because chemisorption reactions of proteins are not selective and many potential anchoring groups may be present on the substrate, mobility requirements are lower. For molecule-molecule interactions, control over hydration is desirable. One way to prevent drying without immersion in water is to work in saturated air. In many printing operations, this is helpful. However, the humidity level is difficult to regulate. Molecules can interact creating adhesion detectable with an adhesion sensor as described, for example, in EP 0 962 759 A1. For example, an antibody and its matching antigen may interact. Similarly, a DNA oligomer may hybridize with its complementary oligomer.
Other printing technologies include Ultra Violet (UV) lithography or UV-molding. In such techniques, a patterned glass master is pressed into a liquid prepolymer. The prepolymer is then cured and solidified by exposure to UV light. See, for example, M. Colburn et al., “Patterning nonflat substrates with a low pressure, room temperature imprint-process”, J. Vac. Sci., Technol. B. 6, 2161 (2001). On release, the pattern formed in the polymer is a replica of the master. However, it is difficult to displace such a polymer on large areas to achieve a pattern with acceptable definition. There is usually a residual layer left. Use of an identically patterned elastomeric stamp in place of glass provides similar replication except for two differences, as follows. Experiment indicates that in protruding areas of the stamp, where the polymer was to be displaced down to the surface, localized dome-like protrusions of trapped material were discovered. Secondly, variation was observed in the thickness of features molded from the recesses in the stamp Typically, the thickness of each feature was smaller in its center. The depth of depression was proportional to the load applied to the stamp. See, for example, Bietsch and Michel, “Conformal contact and pattern stability of stamps used for soft lithography”, J. Appl. Phys. 88, 4310 (2000); Johnson, “Contact Mechanics”, Cambridge University Press, Cambridge (1985); and S. P. Timoshenko and J. N. Goodier, “Theory of Elasticity”, Mc-Graw-Hill, New York). Formulae for the displacement of liquids can be derived from lubrication theory. See, for example, A. Cameron, “Basic Lubrication Theory” Wiley, New York (1981)).
Hydrogels are used in gel electrophoresis. Because hydrogels are flexible, they are also used as stamp materials for printing of biological molecules. See, for example, D. Brett et al., Langmuir 14, 3971 (1998) and Langmuir 16, 9944 (2000); M. A. Markowitz et al., Appl. Biochem. and Biotechnol. 68, 57 (1997). Hydrogels are mainly composed of water, and water easily diffuses through a hydrogel matrix. Thus, hydrogels avoid hydration problems associated with PDMS based printing. However, hydrogel stamps change volume on exposure to water or upon drying. Also, molecules can diffuse between protrusions of the stamp. Hydrogel stamps for parallel printing of different molecules with good registry and separation among the spots have yet to be demonstrated.
Printing of biological molecules from an affinity stamp with catalysts and of hydrophilic molecules from a hydrophilized PDMS stamp onto a substrate have both been demonstrated on a research level but are more difficult to implement commercially where areal transfer over large surfaces is desired. The difficulty arises either because there is not enough of the third medium required for hydration chemisorption, or hybridization on the substrate or because there is too much third medium, preventing intimate contact and transfer. Third medium herein is the general expression for a medium in which other components are carried. Depending on the application, the third medium may be a gas, water, solvent, or polymer. See, for example, A. Bernard et al., “Affinity capture of proteins from solution and their dissociation by contact printing”, Nature Biotechnol. 19, 866 (2001).
A third medium in the form of damping water is found in offset printing of viscous inks onto impermeable substrates. See, for example, J. M. Adams, D. D. Faux and J. J. Rieber, “Printing Technology 4th Ed.” Delamare Publishers, Albany, N.Y., 1996. Offset printing typically employs a printing cylinder having a rubber printing surface. Prior to application of ink, the surface is moistened. This transfers a thin layer of detergent carrying water to the printing surface. The detergent reduces surface tension in the water. The water layer covers the surface but can be displaced by application of patterned link. A water layer improves definition in printing processes where information on a master is presented as a wettability pattern. The water layer prevents incursion or adherence of ink to ink repelling regions. In transfer from the printing surface to paper, water is absorbed into the fiber mesh of the paper and dried. This process does not work on impermeable materials. In such cases, the printing rubber surface slips on the water layer and the pattern is smeared. A conventional solution to this problem is to roughen the surface to be printed and render it hydrophilic. By controlling the thickness of the water layer, fluid transport over large areas can be prevented. This avoids need for capillary channels that obstruct printing of pictures. Roughening also determines fluid resistance in percolating channels and therefore determines printing speed. Roughening creates a random distribution of peaks and troughs. These lead to unobstructed percolation path between larger zones. The random process is however inefficient because it also creates many disconnected capillary paths.
A third medium also affects high speed contact between a rigid object and an adhesive tape in a gas such as air. The gas can build considerable pressure between the object and the tape. The pressure deforms the tape to create a central depression. The depression causes trapping of an air pocket. The air pocket prevents accurate positioning of the object in subsequent process steps such as pick and place operation in the manufacture of semiconductor subassemblies, disk read/write heads, and the like. Such assembly is increasingly important as semiconductor technology moves from creating entire processors on one chip towards assembling sub-components on intermediate carriers. To assemble and process several chips in parallel, in flip chip bonding for example, typically requires pre-assembly on an adhesive tape or pad.
Self-assembly of μm-sized components on a chemically patterned surface in a third medium is typically a slow process in which particles approach the target surface closely enough to allow specific molecular or chemical interactions. Typically, such a process requires vigorous agitation to provide particles with sufficient diffusion through the third medium to establish contact with counterparts on the surface. It can be difficult to separate the particles when the third medium is not present. For assembly, it is desirable to have an intermediate interaction between the parts to be assembled to better control assembly. Appropriate placement produces stronger interaction, while inappropriate placement provides produces weaker interaction. For faster and more predictable assembly, an improved approach process for micrometer to millimeter sized particles in a third medium would be desirable. The third medium helps suspend particles that would otherwise affected by gravitational forces.